1. Field of the Invention
The present invention relates to a method for determining the operability of a source range detector disposed in the vicinity of an operating nuclear reactor. In particular, it provides a method of determining the operability of a source range detector, without damaging the detector, while the reactor is operating in the intermediate or power range.
2. Description of the Prior Art
The power level of a nuclear reactor is generally divided into three ranges: the source or start-up range, the intermediate range and the power range. The power level of the reactor is continuously monitored to assure safe control and operation. Such monitoring is conducted by means of neutron detectors placed outside but adjacent to the reactor core for measuring the neutron flux of the reactor. Since the neutron flux at any point in the reactor is proportional to the fission rate, the neutron flux is also proportional to the power level of the reactor.
Fission and ionization chambers have been used to measure neutron flux in the intermediate and power range of a reactor. Such fission and ionization chambers are capable of operation at all normal reactor power levels without damage; however, they are generally not sensitive enough to accurately detect the low level neutron flux emitted in the source range. Thus, separate low level or source range detectors are used to monitor neutron flux when the power level of the reactor is in the source range.
As disclosed in U.S. Pat. No. 4,186,048, it is known to use proportional counters as source range detectors because they are extremely sensitive and hence highly suitable to detect the low energy radiation produced by a nuclear reactor in its start-up range. A typical proportional counter used for such purposes is a Type WL-23706 boron trifluoride (BF.sub.3) gas-filled proportional counter made by Westinghouse Electric Corporation. Such a proportional counter has an electrode structure which includes a fine central wire which acts as the anode and an annular cathode which surrounds the anode.
In operation, incident neutron radiation will liberate electrons from gas atoms within the counter to create electron-ion pairs. A high electric field is produced between the electrodes of the device by applying a corresponding high voltage across the electrodes. As a result, gas multiplication or avalanche gain occurs in the high field region surrounding the wire anode whereby the initial electrons liberated by neutrons collide with other gas atoms to create additional electron-ion pairs in the gas. The resulting charge is swept out of the gas by the electric field, the electrons moving to the anode, and positive ions moving to the cathode. The moving charge causes a current to flow in an external circuit.
When the nuclear reactor is operating at a power level above the source range, it is generally necessary to de-energize the source range detectors since the neutron flux at those operating levels would produce a current in the proportional counter which could melt or severely damage the wire anode. This mode of operation presents a problem in that prior to reducing the reactor power level to the source range, it is not known whether the source range detector is operable.
To date the only method of determining the operability of the above type of source range detector is to wait until the reactor power has decreased to the low intermediate range (which overlaps with the upper end of the source range) and then to turn on the detector to see if it operates. However, this manner of detecting the operability of the source range detector is not entirely satisfactory and can be dangerous because, if the detector is inoperable, there is little if any time to install a spare detector or institute administrative safeguards to insure that the reactor does not inadvertently return to nuclear criticality.
A need thus exists to be able to identify if the source range detector is operable prior to reducing the reactor power to the source range, without damaging the detector in the process.